Cryptography device having secure provision of random number sequences

ABSTRACT

Secure computation of a random number sequence in a cryptographic device. The computation is secured by receiving a homomorphic ciphertext seed vector, selecting an initial internal state from the seed vector, the initial internal state composed of a subset of elements of the seed vector, updating an internal state from a previous internal state using multivariate functions accepting elements of the previous internal state as inputs to produce a homomorphic ciphertext from homomorphic ciphertext input values, generating an intermediate result vector of homomorphic ciphertexts from the homomorphic ciphertext internal state multivariate functions accepting the elements of the internal state as inputs to produce a homomorphic ciphertext from homomorphic ciphertext input values, and decrypting the intermediate result vector elements into plaintext vector elements, thereby producing a plaintext deterministic random sequence vector corresponding to plaintext seed elements used to produce the seed vector. Other systems and methods are disclosed.

BACKGROUND OF THE INVENTION

The present invention generally relates to cryptography devices, andmore particularly to cryptography devices that avoid manipulation ofsecret data, such as keys and random number generator seeds, inplaintext to thereby be more resilient to side-channel attacks includingwhite-box attacks designed to discern such secret data.

Broadly, cryptography provides mechanisms by which a private plaintextmessage may be protected from being divulged by converting the messageinto a ciphertext that may only be deciphered, i.e., converted back intothe plaintext by specific persons or entities that are privy to a secretkey required for performing the deciphering operation.

Two major categories of cryptography are shared secret cryptography andprivate-key-public-key cryptography (herein, simply referred to aspublic key cryptography). The former includes the Digital EncryptionStandard (DES) and the Advanced Encryption Standard (AES). The latterincludes Rivest-Shamir-Adleman (RSA).

In shared secret cryptography the encrypting party and the decryptingparty share a secret key (herein, the shared secret key) that is used toboth encrypt and decrypt a message. In public key cryptography, therecipient of a ciphertext message, i.e., the decrypting party, has aprivate key or secret key required to decipher ciphertext messagesencoded with the public key. In other words, there is an associationbetween a particular private key and a particular public key; they forma key pair. The public key is made available to anyone who wishes tosend an encoded message (a ciphertext message) whereas the correspondingsecret key is kept secret by the intended recipient of messages.

Traditionally, cryptography relied on a message being turned into aciphertext, that only sender and recipient would know the required keys,and that the encryption and decryption processes would not be availablefor a nefarious person trying to discern the secret message. Keys wereprotected by not giving access to the machines that were used to decrypta text. The endpoints of a communication is trusted and thecommunication channel between the endpoints is protected by turningmessages into ciphertext that cannot be decrypted without access to therequired decryption key. This is referred to as black-box cryptography.

However, there are situations where the cryptography device has to bemade available on open devices to a party that not necessarily shouldhave access to the cryptography key. For example, in a digital rightsmanagement (DRM) scenario a publisher may wish to make a DRM protectedwork available to a subscriber. As long as the subscriber satisfies theterms of the subscription, the work is available. However, at the end ofa subscription term, the subscriber should not have access to the work.

The open nature of these systems, which may be referred to as white-boxenvironments—whether PCs, tablets, or smart phones—renders thecryptography software extremely vulnerable to attack because theattacker has complete control of the execution platform and of thesoftware implementation itself. The attacker can easily analyze thebinary code of the cryptography application and, for example, memorypages used for temporary storage during the execution by interceptingsystem calls, tampering with the binary or execution files. Suchmanipulation may, for example, be performed using debuggers and hardwareemulation tools.

These attacks include trace execution, examination of intermediateresults, and access to keys located in memory as well as performance ofstatic analysis on the cryptography software and alteration ofsub-computations for perturbation analysis.

Generally, countermeasures aimed to protect against such attacks in awhite-box environment are referred to as white-box countermeasures.

If the work is protected through cryptography, the decryption key may beprovided on the subscriber's cryptography device, e.g., a mobile devicesuch as a mobile telephone, in a manner that it can be used by thedevice to decrypt the work without revealing either the key or thealgorithm to the subscriber. The key might be hidden in some way insidethe code implementing the decryption algorithm may be obfuscated so thatit is very difficult to determine any information about the value ofkey. Cryptographic countermeasures in a white-box environment arereferred to as white-box cryptography.

White-box cryptography was first described by Chow et al. in [Chow AES]Stanley Chow, et al., White-Box Cryptography and an AES Implementation,in Proceedings of the 9^(th) International Workshop on Selected Areas inCryptography (SAC 2002), volume 2595 of Lecture Notes in ComputerScience, pp. 250-270. Springer, 2002 and in [Chow DES] Stanley Chow, etal., White-Box Cryptography DES Implementation for DRM applications, inProceedings of the ACM Workshop on Security and Digital RightsManagement (DRM 2002), volume 2696 of Lecture Notes in Computer Science,pp. 1-15. Springer, 2002. [Chow AES] and [Chow DES] are bothincorporated herein by reference in their entireties.

However, hitherto, all practical white-box cryptography approaches havebeen broken. Therefore, there is still an unmet need to providecryptography devices that protect cryptographic keys from beingdivulged.

Random numbers are frequently used in cryptographic operations. Forexample, to introduce a bit more obfuscation into a calculation, thecalculation may introduce a random value. Another example in whichrandom numbers are used is ElGamal encryption. ElGamal is an example ofpublic key encryption. To create a public key, the key generator selectsa random number. Many algorithms involving random number sequences,require the sequences to be deterministic, i.e., reproducible.

Other uses of random numbers in white-box cryptography includechallenge-response protocols in which the challenge is created using arandom number, the generation of nonce vales in ECDSA (Elliptic CurveDigital Signature Algorithm) signatures, random number masks (inwhite-box countermeasures), and random padding of signatures andencrypted values.

Pseudo random number generators typically operate using a seed value.See e.g., Elaine Barker and John Kelsey, Recommendation for RandomNumber Generation Using Deterministic Random Bit Generators, NISTSpecial Publication 800-90A, NIST, January 2012. That seed valuepresents a vulnerability to the underlying cryptographic scheme.

From the foregoing it will be apparent that there is still a need forimproving the security of devices that rely on white-box cryptography byproviding a secure mechanism for generating deterministic pseudorandomnumber sequences.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a mobile device optionallyconnected over a network to one or more servers from which the mobiledevice may obtain materials which it would perform cryptographicoperations on, for example, for obtaining access to digital rightsmanagement protected content.

FIG. 2 is a schematic illustration of a cryptography device, e.g., amobile device such as a mobile telephone or tablet.

FIG. 3 is a schematic illustration of programs and data stored in amemory of the cryptography device of FIG. 2.

FIG. 4 is a high-level schematic illustration of a mobile device havinga white-box cryptography module program listing that may be stored inthe memory of a cryptography device as illustrated in FIG. 3 and whichperforms a cryptographic operation, e.g., an Advanced EncryptionStandard (AES) decryption, to obtain access to a message obtained from aserver.

FIG. 5 is a diagram illustrating basic concept of pseudorandom sequencegeneration.

FIG. 6 is a flowchart of an exemplary deterministic pseudorandom numbergenerator.

FIG. 7 is a diagram illustrating the generation of a random numbersequence using n+p multivariate quadratic polynomials such that thenfirst polynomials have quadratic monomials and no linear terms.

FIG. 8 is a diagram illustrating an alternative embodiment forgeneration of a random number sequence using n+p multivariate quadraticpolynomials such that the n first polynomials are multivariate quadraticpolynomials with or without linear terms.

FIG. 9 is a diagram illustrating a second alternative embodiment forgeneration of a random number sequence.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention. It is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein in connection with one embodiment may beimplemented within other embodiments without departing from the spiritand scope of the invention. In addition, it is to be understood that thelocation or arrangement of individual elements within each disclosedembodiment may be modified without departing from the spirit and scopeof the invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to which the claims are entitled. Inthe drawings, like numerals refer to the same or similar functionalitythroughout the several views.

In an embodiment of the invention, a cryptographic device, e.g., amobile telephone, a tablet, or a personal computer executes a white-boxcryptography mechanism incorporating a deterministic random numbergenerator in which the random number generator seed value is notmanipulated in plaintext on the cryptographic device.

FIG. 1 is a schematic illustration of a network 111 connecting acryptographic device 103, e.g., a mobile telephone, a tablet, or apersonal computer, to one or more remote servers 113. The cryptographicdevice 103 is operated by a user 101 who interacts with one of theservers 113 via a web browser window 105 of a web browser. In theexample scenario illustrated in FIG. 1, the cryptographic deviceprovides the cryptographic operations on behalf of the user 101, e.g.,to decrypt messages, e.g., message C 115, which may correspond to aplaintext message M, and which is received from the remote server 113 a.The messages may be a DRM protected content, such as a computer programor a music library. While not always used in such scenarios,deterministic random number generation is used in some protocols, forexample, in challenge-response protocols in which the challenge isgenerated using a pseudo random generator.

While FIG. 1 provides an illustration of one scenario in whichcryptography may play an important role; there are many other importantuses for cryptography. Thus, the technology described herein is notlimited in its application to the usage example illustrated in FIG. 1.

In one embodiment, discussed in conjunction with FIGS. 5 through 9, acryptographic device 103 receives an encrypted random-number-generationseed value from a secure environment 701 (FIGS. 7-9) and generates anencrypted random number sequence from the encrypted seed and decryptsthe result to obtain a decrypted random number sequence as if the randomnumber sequence had been generated from the plaintext seed correspondingto the encrypted seed value.

FIG. 2 is a schematic illustration of a cryptography device 103, forexample, a mobile telephone, tablet, or personal computer. Thecryptography device 103 may include a processor 201 connected via a bus202 to a random access memory (RAM) 203, a read-only memory (ROM) 204,and a non-volatile memory (NVM) 205. The cryptography device 103 furtherincludes an input/output interface 207 for connecting the processor 201,again typically via the bus 202, to a connector 211 by which thecryptography device 103 may be connected to an antenna 211 by which thecryptography device 103 may connect to various forms of wirelessnetworks, e.g., wide-area networks, WiFi networks, or mobile telephonynetworks. Alternatively, the cryptography device 103 may connect tonetworks via wired network connections such as Ethernet.

The ROM 204 and/or NVM 205 may include computer programs 301 as isillustrated in FIG. 3. While it is here depicted that the computerprograms 301 are all co-located in the ROM 204 or the NVM 205, in actualpractice there is no such restriction as programs may be spread out overmultiple memories and even temporarily installed in RAM 203.Furthermore, the portable security device 109 itself may includemultiple ROMs or NVMs. The programs 301 include operating systemprograms 219 as well as application programs loaded onto thecryptography device 103.

In a preferred embodiment, the programs include a white-box cryptographymechanism 213. While depicted in FIG. 3 as a distinct module 213, inmany implementations a white-box cryptography mechanism may beimplemented as a number of tables, which, for the purpose ofobfuscation, may be scattered about in the memory 205. Suchimplementation details are outside the scope of this document.

The cryptography mechanism 213 of the cryptography device 103,implements one or more cryptography functions (CF) 215, which may beimplemented as a number of computation blocks 217.

The ROM 204 or NVM 205 may also contain private data, such as a secretkey 221, stored either in its basic form or in derived quantities. Whilein many white-box cryptography mechanisms the shared secret is stored onthe cryptography device 103, in a preferred embodiment, the sharedsecret is not stored on the cryptography device 103. The details of thesecret key 221 stored and used in conjunction with the white-boxcryptography mechanism 213 is described in greater detail below.

Thus, the cryptography device 103 may receive a document, a message, oran encrypted program as the encrypted message C 115 via the connector211. The processor 201, by executing instructions of the cryptographymodule 213, may decrypt the document/message using the secret key 221according to the mechanism described herein below.

FIG. 4 is a high level schematic illustration illustrating a prior artembodiment white-box cryptography mechanism. A service provider 413(analogous to the service provider 113 of FIG. 1) may provide anencrypted message M 415 to the cryptography device 403 (analogous todevice 103 of FIG. 1). The message 415 is encrypted using a key K 417known to both the service provider 413 and the cryptography device 403.In the case of shared secret cryptography, the key K 417 is a sharedsecret. In the case of public key cryptography, the key 417 is apublic/private key pair where the service provider 413 is possession ofthe public key and the cryptography device 403 is in possession of theprivate key.

A white-box cryptography mechanism 419 executing on the cryptographydevice 403 decrypts the message 415 using the shared secret 417 storedon the cryptography device 403. In a prior art white-box cryptographymechanism 419, the cryptography algorithms may be implemented as set oftables stored in memory with the shared secret 417 hidden within thesetables.

Patent application entitled “Cryptography Device Having ImprovedSecurity Against Side-Channel Attacks” of the same inventors describes amechanism for using homomorphic encryption in the context of white-boxcryptography. This mechanism makes use of a deterministic randomsequence to improve the security of the cryptographic processes employedtherein. As the random sequence generation is one factor in providingsecurity to the cryptography mechanism, it is important to protect thesecurity of the random sequence generation mechanism.

Pseudorandom number sequences are typically generated from a seed. FIG.5 is a flowchart illustrating the basics of random sequence generation.The seed value 501 is input to a random sequence generator 503. Therandom sequence generator uses the seed value to generate one or morerandom sequences 505. In other words, one seed value may be used togenerate multiple sequences.

One mechanism for doing so is to provide an internal state 507, which isused by an output function to compute each random sequence 505. Theinternal state 507 is updated by an update function 509. On eachinvocation of the random sequence generator 503, the update function 509generates a new internal state. The seed 501 is used to initialize theinternal state 507.

An output function 511 uses the internal state 507 to compute eachrandom sequence 505.

According to embodiments—illustrated in FIG. 7, FIG. 8, and FIG. 9—theseed, the security of which is important to protecting the overallsecurity of a cryptographic mechanism that uses random sequences, is nothandled in plaintext.

Consider first, without considering cryptographic protection of theseed, an algorithm for generating a deterministic pseudorandom numbersequence without consideration for protection of the seed. One suchalgorithm is illustrated in the flow chart of FIG. 6, which includesfour phases:

-   -   Parameter definition    -   Provisioning    -   Initialization    -   Calls to obtain deterministic random sequence.

Parameter Definition

First, a few parameters may be defined, including n, the number ofelements in the internal state, S, and the p, the number of elements inthe output pseudorandom number sequence, step 601.

Provisioning

Next, in the provisioning phase, n+p multivariate polynomials Q_(j) in nvariables denoted by Q₁(x₁, . . . , x_(n)), . . . , Q_(n+p)(x₁, . . . ,x_(n))=Q₁(x), . . . , Q_(n+p)(x) are defined, step 603. The polynomialsQ are used to (1) update the internal state, S, and to calculate theoutput deterministic random sequence, O, where O is a sequence of ppseudorandom numbers. Each polynomial Q_(j) accepts the n values of theinternal state S_(i) as input parameters.

The n first multivariate polynomials Q_(j) are used to update theinternal state (i.e., to compute S_(i+1)) and the p remainingmultivariate polynomials are used to output a pseudorandom value, O_(i).

While there are many possibilities for the polynomials Q, preferredembodiments include the following cases:

-   -   Case 1. (illustrated in FIG. 7) Q is an n+p multivariate        quadratic polynomials such that the n first polynomials have        quadratic monomials and no linear terms (and the p remaining        polynomials are multivariate quadratic polynomials with or        without linear terms)    -   Case 2. (illustrated in FIG. 8), which is a general case of Case        1, Q is an n+p multivariate quadratic polynomials such that the        n first polynomials are multivariate quadratic polynomials with        or without linear terms (and the p remaining polynomials are        multivariate quadratic polynomials with or without linear terms)

Thus, for both Case 1 and Case 2, for the embodiments of FIG. 7 and FIG.8, respectively, the functions Q₁(x), . . . , Q_(n)(x) have the form:

$Q_{k} = {\sum\limits_{i,j}{\lambda_{i,j}x_{i}x_{j}}}$

For Case 1, the embodiment of FIG. 7, the functions Q_(n+1)(x), . . . ,Q_(p)(x) have the form:

$Q_{k} = {\sum\limits_{i,j}{\lambda_{i,j}x_{i}x_{j}}}$

I.e., for the embodiment of FIG. 7, all the Q_(i) functions have thesame form.

For Case 2, the embodiment of FIG. 8, the functions Q_(n+1)(x), . . . ,Q_(p)(x) have the form:

$Q_{k} = {{\sum\limits_{i,j}{\lambda_{i,j}x_{i}x_{j}}} + {\sum\limits_{i}{\alpha_{i}x_{i}}}}$The coefficients λ_(i,j) and α_(i) are set by a provisioning server,e.g., the secure environment 701 of FIG. 7 and FIG. 8.

The embodiment of FIG. 9, key switch, may be a variant of either Case 1or Case 2. Thus, the form of the Q_(i) functions for that embodimentsdepends on whether it is a variant of Case 1 or Case 2.

Next, a seed vector having 2n values is received, step 605. In analternative embodiment, the provisioning phase may also includeprovisioning the cryptographic device with any required constants. Anexample of the latter is illustrated in FIG. 8

As is discussed in greater detail here in below, in preferredembodiments, the seed vector is encrypted in a secure environment priorto being provisioned to a cryptographic device 103.

In preferred embodiments, the seed vector SD is encrypted using theElGamal homomorphic encryption system, described in [Elgamal] T.Elgamal, A public key cryptosystem and a signature scheme based ondiscrete logarithms, IEEE Transactions on Information Theory (Vol. 31,Issue. 4, July 1985). Details of ElGamal encryption are provided hereinbelow.

Being a homomorphic encryption system, ElGamal provides the mechanism ofallowing computations to be performed on encrypted values such that adecryption of the computation result is the same as the performance ofthe same calculation on corresponding plaintext values. Consider aquantity x, which encrypted in to a value y and a function ƒ(x). If thehomomorphic property holds over f(x), then ƒ(x)=decrypt (y). This allowscomputations to be performed on decrypted values with the computationresult eventually obtained through a decryption operation.

Initialization of the Internal State

From the seed vector, an initial internal state, S₀, is initializedhaving n values, which are selected from the 2n values of the seedvector, step 607. Details of one embodiment for the selection process isdescribed in conjunction with FIG. 7.

Calls to Obtain a Deterministic Random Sequence O_(i)

Internal State Update Function: When the cryptographic device 103receives a call to calculate a new output deterministic random sequenceO, the cryptographic device first updates the internal state S_(i)applies the polynomials Q_(j) to the previous internal state S_(i−1),step 609.

Each internal state member S_(i)[j] is set to the value:S _(i)[j]=Q _(j)(S _(i−1)[1], . . . ,S _(i−n)[n])

Output Function, Step 1: Next, an encrypted output vector O iscalculated, step 611. As discussed herein above, for state i, p outputvalues are produced in the output sequence O. At Step 1 (the encryptedoutput vector), the jth output value O_(i)[j] is determined using then+j th polynomial Q using the current internal state (S_(i)) as inputs:O _(i)[j]=Q _(n+j)(S _(i)[1], . . . ,S _(i)[n])

Output Function, Step 2: As each input value S_(i) is a ciphertextvalue, the members O_(i) of the output vector are also ciphertext. Thesevalues O_(i) are therefore decrypted, step 613, into a plaintext resultV_(i). As discussed in greater detail below, there are several differentoptions for performing the decryption.

FIG. 7 is an illustration of a mechanism for generating a randomsequence in which a seed is provided by a secure environment 701. In anembodiment of the mechanism of FIG. 7 the seed is provided by the secureenvironment 701 and is not manipulated in cleartext by the cryptographydevice 103 thereby reducing the risk of attacks against the seed. Themechanism includes the following steps:

-   -   Encrypting seed values in a secure environment    -   Provisioning cryptographic device with        -   Encrypted seed values        -   Additional constants (Case 2, illustrated in FIG. 8)    -   Selecting initial internal state from seed value using an        initial value    -   Updating the internal state    -   Generating encrypted intermediate output based on the internal        state    -   Decrypting the encrypted intermediate output into a plaintext        deterministic random sequence

Provisioning the Cryptographic Device with Seed Values

In one embodiment, the secure environment 701 may be an applicationserver that provisions the white-box enabled cryptography algorithm,e.g., a server 113 of FIG. 1. A white-box cryptography algorithm 213,e.g., an implementation of FHE protected cryptographic mechanism asdescribed in the patent application “Cryptography Device Having ImprovedSecurity Against Side-Channel Attacks” of the same inventors, requiresthe use of a random sequence and provides an initial value (IV) in acall to getRandom(R), call 706.

As is described hereinbelow, the getRandom( ) function 703, selects aninitial internal state (S₀) from an homomorphically encrypted seed valueSD 721, step 713, provisioned by the secure environment 701, step 720.

The seed value SD may be generated based on some parameters. Forexample, one option is to use another random number generator in asecure environment to create a random value parameter. That randomnumber generator should not be the same as pseudo random numbergenerator of getRandom( ) 703. As discussed in greater detailhereinbelow, the seed value SD is provided in encrypted form and neverused directly in plaintext on the cryptography device 103.Alternatively, the cryptographic device 103 is pre-provisioned by thesecure environment 701 with the seed value SD.

In one embodiment, the encryption of the seed value vector SD relies onhomomorphic encryption, e.g., ElGamal encryption or a modificationthereto (as described herein below). The homomorphic encryption togenerate the seed vector SD may be performed by a homomorphic encryptionprocedure 715.

ElGamal encryption is based on the algebraic concept of a cyclic ringG∈GF(p) of prime order q having a generator g. In preferred embodiments,p is a prime number of at least 2048 bits. Further, in preferredembodiments, the order q is a prime number of at least 200 bits.

The ElGamal secret key is x∈Z_(q)*, i.e., in {1, . . . , q−1}.

The corresponding public key is the value h=g^(x), together with thecyclic ring, G, the generator, g, and the order of the ring, q.

To encrypt a value, the encrypting party selects a second valuer∈Z_(q)*, wherein r is typically a random value, and creates theciphertext c=(c₁, c₂) corresponding to a plaintext message m, by:(c ₁ ,c ₂)=(g ^(r) ,m*h ^(r))

The input to the homomorphic encryption procedure 715 is a set of 2nmessages, messages m=(m₁, . . . , m_(2n)) 717. Each message m_(i) isselected from Z_(q)*.

The seed vector, SD, which the cryptographic device 103 is provisionedwith by the secure environment 701, is a sequence of ciphertexts having2n elements:SD=(SD ₁ ·SD ₂ , . . . ,SD _(2n))where n is an arbitrary number. In practice n may not be very large; apractical upper limit for small cryptography devices may be 32.

In a preferred embodiment each message m_(i) 717 is a static value thatis randomly selected for a particular deployment of the white-boxcryptography mechanism described in FIG. 7, i.e., a particularcryptographic device 103. The secure environment 701 may generate,randomly, the messages m_(i) and link them to a particular app on aparticular cryptography device 103.

The homographic encryption procedure 715 of the secure environment 701,using ElGamal encryption, creates an encrypted sequence SD 719 from themessages m=(m₁, . . . ,m_(2n)) 717 and the public key of thecryptographic device 103, as follows:

SD₁ = (g^(r), m₁ * h^(r)) SD₂ = (g^(r), m₂ * h^(r)) …SD_(2n) = (g^(r), m_(2n) * h^(r))wherein, the ElGamal public key of the cryptographic device 103 is thetuple (h,G,g,q) as described hereinabove.

In general, the ElGamal scheme is homomorphic under multiplication,e.g.,

  SD₁ * SD₂ = (g^(r) * g^(r), (m₁ * h^(r)) * (m₂ * h^(r))) = (g^(2r), m₁m₂h^(2r))(SD₁ * SD₂) * (SD₃ * SD₄) = (g^(2r) * g^(2r), (m₁m₂ * h^(2r)) * (m₃m₄ * h^(2r))) = (g^(4r), m₁m₂m₃m₄h^(4r))

and so on.

While the ElGamal scheme is homomorphic under multiplication, in thegeneral case the ElGamal scheme is not homomorphic under addition.However, when the random exponent r is the same for two ciphertexts, thehomomorphic property holds also for addition in some sense with respectto those two ciphertexts, e.g., the following computed value is acorrect ElGamal ciphertext:(g ^(r),(m ₁ h ^(r)))+(g ^(r),(m ₂ h ^(r)))=(g ^(r),(m ₁ +m ₂)*h ^(r))wherein,(g ^(r),(m ₁ +m ₂)*h ^(r))is an ElGamal ciphertext corresponding to the plaintext of the additionm₁+m₂. For two ciphertexts (g^(r), m₁h^(r)) and (g^(r), m₂h^(r)),multiplication is as follows:(g ^(r),(m ₁ h ^(r)))*(g ^(r),(m ₂ h ^(r)))=(g ^(2r),(m ₁ *m ₂)*h ^(2r))

Thus, if R is the ciphertext exponent, which is updated on eachoperation, then for addition R=r both before and after the additionoperation. However, for multiplication, R=r before the multiplicationoperation and R=2r after the multiplication operation.

In an alternative embodiment, a modified version of ElGamal encryption,which has similar properties to those described above, is used. In thismodified version the following encodings are used:

SDM₁ = (h^(r), m₁ * g^(r)) SDM₂ = (h^(r), m₂ * g^(r)) …SDM_(2n) = (h^(r), m_(2n) * g^(r))

This modified ElGamal encryption scheme allows for implementing atrapdoor key that may be used at the end of the process of transferringthe encrypted seed values to the cryptography device 103, referred to asSDM to differentiate from seed values encrypted using the standardElGamal, i.e., SD.

Preferably, the value of the messages m=(m₁, . . . ,m_(2n)) 717 do notdeliberately contain any redundancy. Rather, it is preferred that thevalues for the messages m=(m₁, . . . ,m_(2n)) 717 are selected randomlybecause if a value m_(i) had some relationship to another value m_(j),an attacker could base an attack on the knowledge of the ratiom_(i)/m_(j). To further strengthen the resilience against attacks basedon the ratio m_(i)/m_(j), the messages m=(m₁, . . . ,m_(2n)) 717 may beconstructed as products of a sequence of random values and a secretelement k, e.g.:m _(i)=randomValue( )*k

The encrypted seed values SD (or SDM) 719 are transferred to thecryptography device 103, step 720. Thus, the cryptography device 103 hasbeen provisioned with the vector (SD=SD₁, . . . , SD_(2n)) 721, whichcorresponds to the SD vector 719.

Additional Provisioning for Case 2

In an alternative embodiment illustrated in FIG. 8 (and referred toherein as Case 2), the cryptography device 103 is provisioned with aconstant vector C 801, step 803, which is used when the outputgeneration function includes linear terms.

Selecting an Initial Internal State (S₀) from a Seed Value Vector and anInitial Value (IV)

To obtain a random sequence, a function, getRandom ( ) 703 is called bythe cryptography device 103 to generate a random sequence based on aninitial value (IV). Specifically, a random sequence consumer 705 callsgetRandom( ), step 706. A random sequence O is returned from getRandom(), step 709.

The random sequence consumer 705, which may be a decryption algorithmcalled from a computation block 217 of the cryptography function 215 ofthe white-box cryptography mechanism 213, calls getRandom( ) 703 with aninitial IV, step 706. IV may be computed from parameters such as time orthe message being encrypted.

The value IV may be passed as an argument to the function getRandom( )703, alternative mechanisms may be used for providing the value IV.

The function getRandom( ) 703 is defined as a Deterministic RandomNumber Generator. In other words, from a specific initial value, IV, areproducible random sequence is produced. That initial value IV is usedin conjunction with a seed value, SD 721, provided externally from thecryptography device 103, e.g., from a secure environment 701.

The getRandom(IV) function 703 uses an internal state S, to (1) computea new internal state, S_(i), 727 by an update function 729 based on thei−1th internal state, S_(i−1), and, and, (2) compute an i-th outputrandom sequence, O_(i) 723, using the updated internals state, S_(i), bya generation function 725. The internal state S_(i) is composed of nciphertexts, S_(i)=(S_(i)[1], . . . , S_(i)[n]). The index i isincremented each time a new output sequence O_(i) is produced, e.g., onsubsequent calls to the function getRandom ( ) 703.

The seed values SD (or SDM) 721 are used by the getRandom(IV) function703 of the cryptography device 103 together with the initial value (IV)711 to compute an initial internal state, S₀ 728, i.e., (S₀[1], . . . ,S₀[n]). The initial state 728 is selected from the 2n SD (or SDM) 721ciphertexts, using the initial value (IV) 711 by:S ₀=(S ₀[1], . . . ,S ₀[n])S ₀=(S ₀[1]=SD _(1+IV) ₁ _(*n) , . . . ,S ₀[n]=SD _(n+IV) _(n) _(*n))(SDM_(i) substituted for SD_(i) in alternative embodiments using themodified ElGamal scheme).

As there are 2^(n) possible values for IV, there are 2^(n) possibleinitial values for the initial state, S₀.

Update of Internal State, S_(i)

Upon each call the getRandom( ) 703, the index counter i for theinternal state S_(i) is incremented, step 707. Thus, on the first call,the internal state is S₁. The index counter is reset when a new initialinternal state S₀ is selected from the SD 721 values, i.e., i is set to0, step 728.

As discussed herein above, the update function 729 and generationfunction 725 are formed by n+p multivariate polynomials in n variablesdenoted by Q₁(x₁, . . . , x_(n)), . . . , Q_(n+p)(x₁, . . . ,x_(n))=Q₁(x), . . . , Q_(n+p)(x). The n first multivariate polynomialsare used to update the internal state (i.e., to compute S_(i) fromS_(i−1)) and the p remaining multivariate polynomials are used to outputa pseudorandom intermediate ciphertext output value, O_(i) 734, whichsubsequently optionally decrypted into a final output value V_(i) 723.

Thus, the update function 729, updates the internal state S_(i)[1], . .. , S_(i)[n] using n polynomials Q_(i)(x₁, . . . , x_(n)), . . . ,Q_(n)(x₁, . . . , x_(n)) of either of the forms described hereinabove,i.e.,

S_(i)[1] = Q₁(S_(i − 1)[1]  …  S_(i − 1)[n]) …S_(i)[n] = Q_(n)(S_(i − 1)[1]  …  S_(i − 1)[n])

Given the ElGamal encryption scheme in which an encryption is in theform (g^(r),m*h^(y)), wherein the random value r was kept constant overthe computation of all the seed value ciphertexts SD_(i) the applicationof the update polynomials Q_(i) changes the value r from one state S_(i)to the next state S_(i+1). Specifically, for the two first updatepolynomial alternatives given above (i.e., polynomials of degree 2), iffor a state S_(i−1) the random factor has the value r, the ElGamalrandom factor for state S_(i) is 2r, whereas for the general case(polynomials of degree d), the random value changes from r to d*r.

Accordingly, the update function 729 tracks the value r for each newinternal state S_(i).

Output Generation Function 725—Step One 733: Compute IntermediateCiphertext Output Values, O

The output generation function 725 is composed of two steps: Step One733, to compute intermediate ciphertext output values, O_(i), 734corresponding to the ultimate plaintext output values, V_(i) 723, andStep Two 735: to optionally or partially decrypt the intermediateciphertext output values O_(i) into the plaintext values V_(i). Step One733 uses the internal state S_(i), which is output from the updatefunction 729, to compute the next ciphertext intermediate output valueO_(i) 734, where O_(i) is a vector of output values; O_(i)=(O_(i)[1], .. . , O_(i)[_(p)]).

Case 1—Monomials of Degree 2 (Illustrated in FIG. 7)

For Case 1, the first output, O₁, has the form:O ₁=(O ₁[1]=(g ² ² ^(r) ,m′ _(1,1) ·g ² ² ^(r)) where,m′ _(1,1) =Q_(n+1)(S ₁[1], . . . ,S ₁[n]). . .O ₁[p]=(g ² ² ^(r) ,m′ _(p,1) ·g ² ² ^(r)) where,m′ ₁[p]=Q _(n+p)(S₁[1], . . . ,S ₁[n]))

For Case 1 (FIG. 7), the ith output, has the form:O _(i)=(O _(i)[1]=(g ² ^(i+1) ^(r) ,m′ _(i)[1]·g ² ^(i+1) ^(r)) where,m′_(i)[1]=Q _(n+1)(S _(i)[1], . . . ,S _(i)[n]). . .O _(i)[p]=(g ² ^(i+1) ^(r) ,m′ ₁[p]·g ² ^(i+1) ^(r)) where,m′ _(i)[p]=Q_(n+p)(S _(i)[1], . . . ,S _(i)[n])

Since the polynomials are monomials of degree 2, it is possible toevaluate the polynomials Q_(n+1)(x₁, . . . , x_(n)), . . . , Q_(n+p)(x₁,. . . , x_(n)) over ciphertext values while maintaining values for thevalue r in the output such that the outputs O_(i), which also areElGamal ciphertexts where O_(i) is the c2 value in a c=(c₁,c₂)=(g^(r),m*h^(r)), have uniform values for r. In other words, at theend of the process, the value of the value r for all the output valuesO_(i) have the same value r and that value r is the same as the value rfor the update function 729.

The index i is the request number, i.e., an ever-increasing counter. Foreach request, the random r as used in the ElGamal encryption isincremented by r_(i+i)=2*r_(i). Therefore, exponent in the ElGamalencryption becomes 2^(i+1)*r as in g² ^(i+1) ^(r), m′_(i)[p]*g² ^(i+1)^(r), for example.

Case 2: Multivariate Quadratic Polynomials (Illustrated in FIG. 8)

In Case 2, the polynomials Q_(i) used by the output generation function733′ are multivariate quadratic polynomials, e.g., for each variablethere is a quadratic term and a linear term.

Consider the ElGamal encryption format:c=(c ₁ ,c ₂)=(g ^(r) ,m*h ^(r))

For illustrative purposes, let's use here R in lieu of r:c=(c ₁ ,c ₂)=(g ^(R) ,m*h ^(R))

The linear term of each polynomial Q_(i) can be computed on ciphertexts.The random value R of the resulting ciphertext is the same as the randomvalue r of the input. On the other hand, the quadratic term of everypolynomial, also computed on the ciphertexts, have a random value thatis twice the random r of the input and twice the value for the linearterm, i.e., the exponent is 2R=2r.

Therefore, for i=n+1, . . . , n+p, we have:Q _(i)(x ₁ , . . . ,x _(n))=A _(i)(x ₁ , . . . ,x _(n))+B _(i)(x ₁ , . .. ,x _(n))

where A_(i) is a polynomial with monomials of degree 2 only and B_(i) isa linear polynomial. The partial evaluation of the polynomial Q_(j)(x₁,. . . ,x_(n)) over the state S_(i) provides two ciphered values:(g^(2R), A_(i)(S_(i)[1], . . . , S_(i)[n])*h^(2R)) and (g^(R),B_(i)(S_(i)[1], . . . , S_(i)[n])*h^(R))

At round i+1, the value of R is expected to be 2^(i)r of the inputciphertexts SD_(i) used to select the initial internal state S₀.

In an alternative embodiment, rather than computing (g^(R),B_(i)(S_(i)[1], . . . , S_(i)[n])*h^(R)), a slightly different value iscomputed. Specifically, consider the knowledge of the ciphertext value(g^(R), B_(i)(S_(i)[1], . . . , S_(i)[n])*h^(R)), the counter i, and aconstant “ciphertext” C_(i)=(g² ^(i) ^(r), (constant_(i)*h^(r))² ^(i) ).From those values, it is possible to compute(B _(j)(S _(i)[1], . . . ,S _(i)[n])*h ^(R))*((constant_(i))*h ^(r))²^(i)then, after the conclusion of Step One 733′ of the output generationstep 735′, for Case 2 (FIG. 8), for the first generation of anintermediate ciphertext output vector, i=1:

O₁[1] = (g^(2²r), m₁^(′)[1] * g^(2²r))  where  m₁^(′)[1] = A_(n + 1) (S₁[1] , …  , S₁[n]) + B_(n + 1)(S₁[1] , …  , S₁[n]) * constant₁  …O₁[p] = (g^(2²r), m₁^(′)[p] * g^(2²r))  where  m₁^(′)[p] = A_(n + p) (S₁[1] , …  , S₁[n]) + B_(n + p)(S₁[1] , …  , S₁[n]) * constant₁

For Case 2, For an update i, output values are as follows:

O_(i)[1] = (g^(2^(i + 1)r), m_(i)^(′)[1] * g^(2^(i + 1)r))  where  m_(i)^(′)[1] = A_(n + 1) (S_(i)[1] , …  , S_(i)[n]) + B_(n + 1)(S_(i)[1] , …  , S_(i)[n]) * constant_(i)  …O_(i)[p] = (g^(2^(i + 1)r), m₁^(′)[p] * g^(2^(i + 1)r))  where  m₁^(′)[p] = A_(n + p) (S_(i)[1] , …  , S_(i)[n]) + B_(n + p)(S_(i)[1] , …  , S_(i)[n]) * constant_(i)where, the values constant_(i) are related to the constants C₁, . . . ,C_(n) 801 are received by the cryptography device 103 from the server,transmission 803 (FIG. 8). In a preferred embodiment, the multiplicationwith the constant values C₁, . . . , C₂ 801 is performed using anobfuscated function.

The above operations are on ElGamal ciphertexts. Therefore, thesequential operations track the value of the random value r used inElGamal encryption. If we denote R as being the value of the randomvalue r for the resulting ciphertext, the relationship between R and ris as follows:

-   -   If the homomorphic operation is multiplication:        (g ^(r) ,m ₁ *h ^(r))*(g ^(r) ,m ₂ *h ^(r))=(g ^(2r) ,m ₁ m ₂ *h        ^(2r))    -   Therefore, R=2r    -   If the homomorphic operation is addition:        (g ^(r) ,m ₁ *h ^(r))*(g ^(r) ,m ₂ *h ^(r))=(g ^(r) ,m ₁ m ₂ *h        ^(r))    -   Therefore, R=r

Output Generation Function, Step Two 735: Decryption of the Output

The output of Step One 733 is computed using ElGamal encrypted values asits inputs. Therefore, the outputs O_(i), which may be consideredintermediate output values, from Step One 733 are also ElGamalciphertexts. These outputs are optionally or partially decrypted orsubjected to other processing in Step Two 735.

There are three different alternative approaches, which may be selecteddepending on the requirements of a particular intended use of theoutput:

1. The output values are decrypted within the Random Sequence Generator703 using a Fully Homomorphic Encryption Scheme or a SomewhatHomomorphic Encryption Scheme. Of course, the decryption must match theencryption scheme used for the inputs to Step One 733.

2. The output values O_(i) from Step One 733 are re-encrypted using atrapdoor re-encryption mechanism. Re-encryption is described hereinabove in conjunction with FIG. 6 in which a ciphertext encrypted usingfully homomorphic encryption n is re-encrypted for a new key. Suchmechanisms are described in [BBS] Blaze, M., Bleumer, G., Strauss, M.:Divertible protocols and atomic proxy cryptography. EUROCRYPT 1998(incorporated herein by reference).

3. The value is partially decrypted using a distributed decryptionmechanism.

Consider these in turn.

Case 1: Local Decryption

One possibility is to use F(S)HE (Fully (Somewhat Fully) HomomorphicEncryption) to perform the decryption step. The data does not need to beexpressed in a bitwise manner since only multiplication, andaddition/subtraction are needed. Then, better performances than in thegeneral case can be achieved.

In that case, the final step is the decryption of the F(S)HE schemewhich means that the decryption key has to be embedded and obfuscated inthe code. A “tracer” can be used to avoid unexpected decryption (asdescribed herein above).

Case II: Re-encryption with Alternate Key

In this embodiment, the getRandom( ) function 703 has a trapdoor key.However, the getRandom( ) function 703 does not have the key fordecryption. In one embodiment, the technique of Blazer, Bleumer, andStrauss (BBS) ([Blaze] Matt Blaze, G. Bleumer, and M. Strauss.Divertible protocols and atomic proxy cryptography. In Proceedings ofEurocrypt '98, volume 1403, pages 127-144, 1998 (incorporated herein byreference)) is used to re-encrypt the output series O from one secretkey to another without manipulation of the series O in plaintext.

BBS is based on the ElGamal cryptosystem and includes the notion of a“re-encryption key” RK_(A→B). Using RK_(A→B), the getRandom( ) function703 can re-encrypt from one secret key to another without ever learningthe plaintext.

G is an algebraic ring of prime order q and having a generator g.

SK_(A)=aϵZ_(q)*randomly selected.

PK_(A)=g^(a)

SK_(B)=bϵZ_(q)*randomly selected.

PK_(B)=g^(b)

RK_(A→B)=b/a=b*a⁻¹ mod q

Encryption of mϵG with random rϵZ_(q)*:c_(A)=(g^(ar), m*g^(r))

Re-encryption using c and RK_(A→B):

$C_{B} = {\left( {\left( g^{ar} \right)^{{RK}_{A\rightarrow B}},{m*g^{r}}} \right) = {\left( {\left( g^{ar} \right)^{\frac{b}{a}},{m*g^{r}}} \right) = \left( {g^{br},{m*g^{r}}} \right)}}$

where m is one of the output messages O_(i)[j].

Thus, the intermediate output values O_(i) [j] is a ciphertext in themodified version of ElGamal wherein the seed values have been encryptedas:SDM _(i)=(h ^(r) ,m _(i) *g ^(r))

Consider, the SDM_(i), values to have been encrypted with the secret keySK_(A), then:O _(i)[j]=(g ^(ar) ,m _(i)[j]*g ^(r))

In an alternative embodiment, the re-encryption scheme uses themechanism described by [AFGH] Ateniese, Fu, Green & Hohenberger,Improved Proxy Re-Encryption Schemes with Applications to SecureDistributed Storage, ACM Transactions on Information and System Security(TISSEC), Volume 9 Issue 1, February 2006, Pages 1-30 (incorporatedherein by reference). The [AFGH] scheme is based on bilinear maps. Onemain advantage of the [AFGH] scheme is that the trapdoor isunidirectional whereas the trapdoor of [BB S] is bidirectional. Anotheradvantage of the [AFGH] scheme is that it is collusion-resistant.

Case III: Distributed Decryption

There are mechanisms for distributing a decryption key x over d partiessuch that decryption requires the cooperation of all (or a subset) ofthe parties. In one example of this distributed decryption technique, todecrypt the outputs O_(i) from Step One 733 relies on the following:x=x ₁ +x ₂ + . . . +x _(d)h=h ₁ *h ₂ * . . . *h _(d) =g ^(x) =g ^(x) ¹ *g ^(x) ² * . . . *g ^(x)^(d)c=(g ^(r) ,m*h ^(r))where, c is one of the output values O_(i).

The secret key x is defined as x=x₁+x₂+ . . . x_(d) and thecorresponding public key h is defined as h==g^(x)×g^(x1)·g^(x2) . . .g^(xd)=h₁*h₂* . . . *h_(d)

If c=(c₁,c₂)=(g^(R), h^(R)*m), one device (for example the first one) inthe distributed decryption computes a partial decrypted ciphertext:c′=(c₁, c₂/(g^(R))^(x1))=(c₁, c/(h₁ ^(R))) and returns the partialresult c′.

The output is c′ and the corresponding secret key for c′ is x′=x₂+ . . .x_(d). c″=(c′₁,c′₂/(g^(R))^(x2)). The next decryption device computesits portion of the decryption: c″=(c′₁, c′₂/(g^(R))^(x) ² ).

The final decryption result, after all the decryption devices haveperformed their partial decryptions is the plaintext m.

Thus, one party, e.g., the cryptographic device 903 may request severalservers to perform the partial decryptions based on those servers'respective portion of the key, respectively. After all the servers havebeen made to decrypt, the final result is obtained.

Case IV: Decryption with Key Switch.

In an alternative to Case I (wherein the output is decrypted within theboundary of the deterministic random number generator (in FIG. 7, thegetRandom( ) function), the homomorphic encryption is replaced with akey switch option illustrated in FIG. 9.

The Provisioning phase (FIG. 6) includes an additional step ofgenerating, in the secure environment 701, a set of candidate keys(CandidateKeys), step 901, from which a subset is selected for computingupdated output values, as described below. The set of candidate keys aredefined as:CandidateKeys={(h ^(y) ¹ ,g ^(y) ¹ ), . . . ,(h ^(y) ^(l) ,g ^(y) ^(l))}

The CandidateKeys set is provided to the cryptography device 103, step903, and stored thereon, 905.

During the Execution phase a subset, CandidateKeysSubset, is selectedfrom the CandidateKeys set, step 907. The subset contains at least twoelements. The cryptographic device 103 also selects a coefficient set{a_(i)}, one coefficient for each element in the CandidateKeysSubset.

From the CandidateKeysSubset two key switch factors, Δhy and Δgy arecomputed, step 909, as follows:

${\Delta\;{hy}} = {\prod\limits_{{{elements}\mspace{14mu}{yi}\mspace{14mu}{in}}{CandidateKeysSubset}}\left( h^{yi} \right)^{a_{i}}}$${\Delta\;{gy}} = {\prod\limits_{{{elements}\mspace{14mu}{yi}\mspace{14mu}{in}}{CandidateKeysSubset}}\left( g^{yi} \right)^{a_{i}}}$

The output values O_(i)[j] 911 are obtained in the same manner asillustrated in and discussed herein above in conjunction with FIG. 7,step 733 or FIG. 8, step 733′ and steps that precede 733/733′. Note, thegeneral case for O_(i)[j] isO _(i)[p]=g ² ^(i+1) ^(r) ,m′ _(i)[p]*h ² ^(i+1) ^(r))

An updated output value O′_(i)[j] is computed, step 913:O _(i)[p]=g ² ^(i+1) ^(r) *Δgy,m′ _(i)[p]*h ² ^(i+1) ^(r) *Δhy)

The corresponding secret key is:key=x+Σa _(i) y _(i)

The updated O′_(i)[j] values are decrypted using the secret key, key,step 915, to produce the final output V_(i).

From the foregoing, the improvement of the security of a cryptographydevice operating in a white box environment and storing secret material,specifically, the seed for a deterministic random number generator, isapparent. This improvement to cryptography devices is provided byenabling the cryptography devices to use homomorphic encryption toperform cryptographic operations requiring random number sequences in amanner that does not use the seed to the random number generator in aplaintext format.

The method is described herein above as using ElGamal encryption.However, in an alternative embodiment another homomorphic encryptionscheme, e.g., the Fully Homomorphic Scheme of Gentry, as described ingreater detail in Craig Gentry, Fully Homomorphic Encryption Using IdealLattices, in Proceedings of the forty-first annual ACM symposium onTheory of computing (STOC '09), pp. 169-178. ACM, 2009.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The invention islimited only by the claims.

The invention claimed is:
 1. A method for operating a cryptographicdevice to secure computation of a deterministic random number sequencein the cryptographic device, comprising: receiving a homomorphicciphertext seed vector (SD) wherein each element of the seed vector (SD)is encrypted from plaintext elements (m) according to a homomorphicencryption mechanism from a secure environment; selecting an initialinternal state (S₀) from the homomorphic ciphertext seed vector (SD)wherein the initial internal state is composed of a subset of elementsselected from the homomorphic ciphertext seed vector (SD); updating aciphertext internal state (S_(i)), which is a vector of homomorphicciphertext elements, from a previous ciphertext internal state (S_(i−1))using update functions that are multivariate functions each acceptingciphertext elements of the previous internal state as inputs and thateach produce a homomorphic ciphertext from homomorphic ciphertext inputvalues; generating an intermediate result vector (O) of homomorphicciphertexts from the homomorphic ciphertext internal state (S_(i)) usingoutput generation functions that are multivariate functions acceptingthe elements of the internal state as inputs and that each produce ahomomorphic ciphertext from homomorphic ciphertext input values, anddecrypting the homomorphic ciphertext intermediate result vector (O)elements into plaintext vector elements, thereby producing a vector (V)of plaintext elements representing deterministic random number sequencecorresponding to the plaintext seed elements (m) used to produce thehomomorphic ciphertext seed vector (SD, SDM).
 2. The method of claim 1,wherein decrypting the homomorphic ciphertext intermediate result vectorcomprises partially decrypting the homomorphic ciphertext intermediateresult vector (O) elements into plaintext vector elements on each of aplurality of decryption servers, combining results of the partialdecryption from the plurality of decryption servers, thereby producingthe vector (V) of plaintext elements representing deterministic randomnumber sequence corresponding to the plaintext seed elements (m) used toproduce the homomorphic ciphertext seed vector (SD, SDM).
 3. The methodof claim 1 wherein the initial internal state is selected using aninitial value (IV) to determine which elements of the homomorphicciphertext seed vector to select into the initial internal state.
 4. Themethod of claim 3 wherein the update and generation functions aremultivariate quadratic monomials without linear terms.
 5. The method ofclaim 3 wherein the update and generation functions are multivariatequadratic polynomials with or without linear terms the method furthercomprising receiving constants (C) from the secure environment, whereinthe method further comprises determining the linear terms from theconstants (C) received from the secure environment.
 6. The method ofclaim 3 wherein the homomorphic ciphertext of the seed vector isproduced using an ElGamal encryption scheme.
 7. The method of claim 6wherein the ElGamal encryption scheme includes an exponent r definingeach ciphertext value c=(c₁, c₂)=(g^(r),m*h^(r)) wherein g is agenerator for a cyclic ring G, h is a public key, and m is a plaintextmessage corresponding to the ciphertext value c, wherein the methodfurther comprises updating the value r for update and generationfunctions such that update and generation results are ciphertexts withknown values for r.
 8. The method of claim 3 wherein the homomorphicciphertext of the seed vector is produced using a modified ElGamalencryption scheme.
 9. The method of claim 8 wherein the modified ElGamalencryption scheme includes an exponent r defining each ciphertext valuec=(c₁, c₂)=(h_(r),m*g^(r)) wherein g is a generator for a cyclic ring G,h is a public key, and m is a plaintext message corresponding to theciphertext value c, wherein the method further comprises updating thevalue r for update and generation functions such that update andgeneration results are ciphertexts with known values for r.
 10. Themethod of claim 3 further comprising reencrypting the homomorphicciphertext intermediate result vector elements using an alternate publicencryption key and decrypting the homomorphic ciphertext intermediateresult vector elements on the cryptographic device using thecorresponding secret encryption key.
 11. The of claim 1 furthercomprising distributing a secret decryption key over a plurality ofdecryption servers and decrypting the homomorphic ciphertextintermediate result vector elements by iteratively decrypting thehomomorphic ciphertext intermediate result vector elements on each ofthe plurality of decryption servers.
 12. The method of claim 3 furthercomprising selecting a subset of candidate keys from a set of candidatekey pairs, compute delta values for the secret and public keys of eachselected candidate key pair, compute updated values for the homomorphicciphertext intermediate result vector elements, computing an updatedsecret key corresponding to the updated values, and decrypting theupdated values using the updated secret key.
 13. A cryptographic deviceproviding secure computation of a deterministic random number sequencein the cryptographic device, comprising: a processor; and an instructionstorage connected to the processor and storing instructions executableby the processor, the instructions including instructions to cause theprocessor to: receive a homomorphic ciphertext seed vector (SD) whereineach element is encrypted from plaintext elements (m) according to ahomomorphic encryption mechanism from a secure environment; select aninitial internal state (S₀) from the homomorphic ciphertext seed vector(SD) wherein the initial internal state is composed of a subset ofelements selected from the homomorphic seed vector; update a ciphertextinternal state (S_(i)), which is a vector of homomorphic ciphertextelements, from a previous ciphertext internal state (S_(i−1)) usingupdate functions that are multivariate functions each acceptingciphertext elements of the previous internal state as inputs and thateach produce a homomorphic ciphertext from homomorphic ciphertext inputvalues; and generate an intermediate result vector of homomorphicciphertexts from the homomorphic ciphertext internal state (S_(i)) usingoutput generation functions that are multivariate functions acceptingthe elements of the internal state as inputs and that each produce ahomomorphic ciphertext from homomorphic ciphertext input values; anddecrypt the homomorphic ciphertext intermediate result vector (O)elements into plaintext vector elements, thereby producing a vector (V)of plaintext elements representing deterministic random number sequencecorresponding to the plaintext seed elements (m) used to produce thehomomorphic ciphertext seed vector (SD, SDM).
 14. The cryptographicdevice of claim 13, the instructions to decrypt the homomorphicciphertext intermediate result vector (O) further includes instructionsto cause the processor to: cause partial decryption of the homomorphicciphertext intermediate result vector (O) elements into plaintext vectorelements on each of a plurality of decryption servers, combine resultsof the partial decryption from the plurality of decryption servers,thereby producing the vector (V) of plaintext elements representingdeterministic random number sequence corresponding to plaintext seedelements (m) used to produce the homomorphic ciphertext seed vector (SD,SDM).
 15. The cryptographic device of claim 13, wherein the initialinternal state is selected using an initial value (IV) to determinewhich elements of the homomorphic ciphertext seed vector to select intothe initial internal state.
 16. The cryptographic device of claim 13,wherein the update and generation functions are multivariate quadraticmonomials without linear terms.
 17. The cryptographic device of claim13, wherein the update and generation functions are multivariatequadratic polynomials with or without linear terms the method furthercomprising receiving constants (C) from the secure environment, whereinthe method further comprises determining the linear terms from theconstants (C) received from the secure environment.
 18. Thecryptographic device of claim 13, wherein the homomorphic ciphertext ofthe seed vector is produced using an ElGamal encryption scheme.
 19. Thecryptographic device of claim 13, wherein the ElGamal encryption schemeincludes an exponent r defining each ciphertext value c=(c₁,c₂)=(g^(r),m*h^(r)) wherein g is a generator for a cyclic ring G, h is apublic key, and m is a plaintext message corresponding to the ciphertextvalue c, wherein the method further comprises updating the value r forupdate and generation functions such that update and generation resultsare ciphertexts with known values for r.
 20. The cryptographic device ofclaim 13, wherein the homomorphic ciphertext of the seed vector isproduced using a modified ElGamal encryption scheme.
 21. Thecryptographic device of claim 20, wherein the modified ElGamalencryption scheme includes an exponent r defining each ciphertext valuec=(c₁, c₂)=(h^(r),m*g^(r)) wherein g is a generator for a cyclic ring G,h is a public key, and m is a plaintext message corresponding to theciphertext value c, wherein the method further comprises updating thevalue r for update and generation functions such that update andgeneration results are ciphertexts with known values for r.
 22. Thecryptographic device of claim 13, the instructions further includesinstructions to cause the processor to perform the step of: reencryptingthe homomorphic ciphertext intermediate result vector elements using analternate public encryption key and decrypting the homomorphicciphertext intermediate result vector elements on the cryptographicdevice using the corresponding secret encryption key.
 23. Thecryptographic device of claim 13, the instructions further includesinstructions to cause the processor to perform the step of: distributinga secret decryption key over a plurality of decryption servers anddecrypting the homomorphic ciphertext intermediate result vectorelements by iteratively decrypting the homomorphic ciphertextintermediate result vector elements on each of the plurality ofdecryption servers.
 24. The cryptographic device of claim 13, theinstructions further includes instructions to cause the processor toperform the step of: selecting a subset of candidate keys from a set ofcandidate key pairs, compute delta values for the secret and public keysof each selected candidate key pair, compute updated values for thehomomorphic ciphertext intermediate result vector elements, computing anupdated secret key corresponding to the updated values, and decryptingthe updated values using the updated secret key.